Flexible photovoltaic device

ABSTRACT

A flexible photovoltaic device includes a flexible substrate and a photovoltaic device body. The flexible substrate is a metal foil or film provided with an electric insulating layer of a material having an electric conductivity of not more than 10 -7  (Ω·cm.) -1  during of light impingement and selected from a heat resistant polymer, a metal oxide, a crystalline or amorphous silicon compound, and an organometallic compound.

BACKGROUND OF THE INVENTION

The present invention relates to photovoltaic devices, and moreparticularly to flexible photovoltaic devices composed ofnon-single-crystal semiconductors.

Photovoltaic devices such as solar cells and photodetectors are capableof converting solar radiation into usable electrical energy. However,the devices of this type have the large problem to be overcome that thegenerating cost is very large as compared with other means forgenerating electric energy. The main reasons are that the efficiency ofutilizing a semiconductor material constituting the body of the deviceis low and much energy is required in producing the semiconductormaterial. In recent years, it has been proposed to use amorphous siliconas a semiconductor material, and this proposal has been considered as atechnique having a possibility of overcoming the above problems. Theamorphous silicon can be produced inexpensively and in large quantitiesby glow discharge decomposition of silicon compounds such as silane andfluorosilane. The amorphous silicon so prepared has a low averagedensity of localized states in the band gap, namely not more than 10¹⁷eV⁻¹ cm.⁻³, and impurity doping of N type or P type is possible in thesame manner as in the preparation of crystalline silicon.

A representative prior art solar cell using amorphous silicon is oneprepared by forming a transparent electrode on a glass substrate capableof transmitting visible rays, forming a P-type amorphous silicon layer,an undoped amorphous silicon layer and a N-type amorphous silicon layeron the transparent electrode in that order by glow discharge, andproviding an ohmic contact electrode on the N-type layer. If lightenters the P-type layer, undoped layer and N-type layer through theglass substrate and transparent electrode, electron-hole pairs aremainly generated in the undoped layer. They are attracted by the PINjunction internal electric field created by the above-mentioned layersand are collected at the electrodes to generate a voltage between theboth electrodes. In solar cells of such a type, the open-circuit voltageis about 0.8 V, and accordingly the cells are not used as they are as apower source for equipment requiring a large power source voltage.

In order to overcome this drawback, it is proposed to raise the voltageby forming separate plural generating zones on a single substrate andcontacting the respective generating zones in series, as disclosed inJapanese Unexamined Patent Publication (Tokkyo Kokai) No. 107276/1980.However, the solar cells prepared according to this proposal still havethe drawbacks that a large surface area is required due to lowconversion efficiency, and that series connection of multiple devices isnecessary because the voltage per single device is only about 0.8 V.Also, the solar cell has no flexibility due to the use of a rigidsubstrate such as glass. On the other hand, flexible solar cells arerequired in the field of electronic machines because of the developmentof flexible printed boards and film type liquid crystal displays.

A solar cell using a polymer thin film rich in flexibility and heatresistance, such as polyimide, as a substrate is disclosed in JapaneseUnexamined Patent Publication No. 149489/1979. However, the use of thepolymer film as a substrate has the disadvantages that the substrate iscurled during the deposition of the amorphous silicon and flat solarcells are not obtained, and because the substrate is not uniformlyheated due to the deformation during the deposition non-uniformamorphous silicon layers are formed.

It is an object of the present invention to provide a flexiblephotovoltaic device.

A further object of the invention is to provide a flexible and heatresistant photovoltaic device.

Another object of the invention is to provide a flexible, small-sizedphotovoltaic device having a plurality of generating zones on a singlesubstrate.

Still another object of the invention is to provide a flexibleintegrated solar cell capable of generating an increased open-circuitvoltage.

These and other objects of the present invention will become apparentfrom the description hereinafter.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided aphotovoltaic device comprising a substrate including a metal foil and anelectric insulating thin film having an electric conductivity of notmore than about 10⁻⁷ (Ω·cm.)⁻¹ during light impingement, and aphotovoltaic device body of non-single-crystal semiconductors, said bodybeing formed on said electric insulating thin film.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing an embodiment of the photovoltaicdevice of the present invention;

FIG. 2 is a schematic view of an apparatus for forming an amorphous orsemiamorphous semiconductor thin film and a photovoltaic device bodycomposed of the amorphous or semiamorphous semiconductor thin filmlayers of the photovoltaic device of the present invention by a plasmaglow discharge method or a sputtering method;

FIG. 3A is a schematic view illustrating a basic structure of a PINjunction photovoltaic device of the type wherein the light impinges onthe P-layer side;

FIG. 3B is a schematic view illustrating a basic structure of a PINjunction photovoltaic device of the type wherein the light impinges onthe N-layer side;

FIG. 4 is a diagram showing an energy band profile of the PINheterojunction photovoltaic device of the present invention;

FIG. 5A is a schematic plan view showing an embodiment of thephotovoltaic device of the present invention in which a plurality ofgenerating zones provided on a single substrate are connected in series;

FIG. 5B is a vertical cross-sectional view taken along the line X--X ofFIG. 5A; and

FIG. 5C is a vertical cross-sectional view taken along the line Y--Y ofFIG. 5A.

DETAILED DESCRIPTION

The term "non-single-crystal" as used herein comprehends "amorphous","semiamorphous" and "microcrystalline".

The substrate used in the present invention is an electrically insulatedmetal foil, in other words, a metal foil on which an electric insulatingthin film is formed. The substrate is flexible and heat resistant, andaccordingly it does not curl during deposition of non-single-crystalsemiconductors and can be employed in the same manner as conventionalsubstrates such as a glass plate and a stainless steel plate to provideflexible photovoltaic devices.

A foil or thin film of a metal having a Young's modulus of not less than8×10³ kg./mm.² is employed in the present invention as a metal foil forconstituting the flexible substrate. The metal includes, for instance,aluminum, copper, iron, nickel, stainless steel, and the like. Thethickness of the metal foil is selected from 5 μm. to 2 mm., preferablyfrom 50 μm. to 1 mm.

Any material having an electric conductivity of not more than 10⁻⁷(Ω·cm.)⁻¹, preferably not more than about 10⁻⁹ (Ω·cm.)⁻¹, during oflight impingement can be employed as the material of the electricinsulating thin film. For instance, a polymer having flexibility andheat resistance and an inorganic material having an electricconductivity of not more than about 10⁻⁷ (Ω·cm.)⁻¹ can be employed asmaterials of the electric insulating layer of the substrate.

Examples of the flexible and heat resistant polymer thin film used asthe electric insulating layer of the substrate are, for instance,polyimide, polyamide, polyamidimide, phenol resin, thermosettingpolyester, thermoplastic polyester, polyhydantoin, polyparabanic acid,poly-p-xylylene, silicone, cyclized polybutadiene homopolymer andcopolymers, and blends of cyclized polybutadiene. The thickness of thepolymer insulation layer is from 1,000 angstroms to 100 μm., preferablyfrom 1 to 20 μm. The polymer film is formed in a usual applicationmanner. For instance, a varnish is prepared by dissolving a polymer or aprecursor thereof in a solvent, and applied to the surface of the metalfoil by spraying, dipping, coating or printing, and is then dried andhardened by heating, ion bombarding or irradiation of ultraviolet rays,β rays, γ rays or electrom beam.

Examples of the inorganic material film used as the electric insulatinglayer of the substrate are, for instance, a metal oxide such as SiO,SiO₂, Al₂ O₃, TiO or TiO₂, crystalline or amorphous silicon carbide,silicon nitride and silicon carbon nitride, i.e. Si.sub.(1-x) C_(x),Si.sub.(1-y) N_(y), Si.sub.(1-x-y) C_(x) N_(y), the hydrides thereof,i.e. Si.sub.(1-x) C_(x) :H, Si.sub.(1-y) N_(y) :H and Si.sub.(1-x-y)C_(x) N_(y) :H, and the fluorides thereof, i.e. Si.sub.(1-x) C_(x) :F:H,Si.sub.(1-y) N_(y) :F:H and Si.sub.(1-x-y) C_(x) N_(y) :F:H, and anorganometallic compound such as an organosilicon compound, anorganotitanate compound, an organotin compound or an organozirconiumcompound. The metal oxide thin film is obtained by, for instance,sputter deposition or electron beam deposition of a metal oxide,chemical vapor deposition of organometallic compounds, oxidativedecomposition of coatings of organometallic compounds, and an oxidationtreatment such as anodization of aluminum. The above micro-crystallineor amorphous silicon compound thin film can be obtained by sputtering orglow discharge decomposition. In the case of sputtering, the desiredthin film is obtained by employing polycrystalline SiC or Si₃ N₄ as atarget, or by employing Si and graphite, etc. as a target. In the caseof glow discharge decomposition, the desired thin film is obtained by aglow discharge in a mixture of a silicon compound such as silane and acarbon and/or nitrogen compound. In that case, preferably a doping gasis not employed, but the silicon compound may be doped with an impuritysuch as boron or phosphorus so long as the electric conductivity of theobtained thin film is not more than 10⁻⁷ (Ω·cm.)⁻¹, preferably not morethan 10⁻⁹ (Ω·cm.)⁻¹, during of light impingement. An amorphous silicon(a-Si) thin film prepared by sputtering or glow discharge of silane orfluorosilane may also be employed as an insulation layer of thesubstrate, since if a strong light such as sun light impinges on thea-Si layer, the electric conductivity of the a-Si layer increases and itserves as a protective circuit so that the photoelectric current leaks.The thin film of the organometallic compound is formed on a metal foilor film by coating application. The organometallic compounds and theprepolymers obtained by hydrolyzing the compounds are employed. They maybe employed alone or in admixture thereof. The organometallic compoundor the prepolymer, which may be dissolved in a solvent if necessary, isapplied to the metal foil by spraying, dipping, or coating, and is driedand hardened to form a film. The thickness of the inorganic materialfilm used as an electric insulating layer is not particularly limited solong as the metal foil or film is electrically insulated. The thicknessis usually selected from 100 angstroms to 20 μm.

The electric insulating layer may be a combination of the polymer thinfilm contiguous to the metal foil and an inorganic material thin filmcontiguous to a lower electrode of the photovoltaic device to befabricated. In that case, if necessary, at least one intermediate layermade of another material may be further provided between the polymerthin film and the inorganic material thin film. The use of thecombination of the polymer film and the inorganic material film as anelectric insulation layer is advantageous in that there is avoided thelowering of the device characteristics which may occur, in the case ofusing only a polymer film as an insulation layer, during the devicefabrication due to generation of a gas from the polymer. Such gasgeneration causes release at the interface between the insulation layerand the device or occurrence of cracks in the device, and flexibility isimproved by a decrease of the thickness of the inorganic material filmand moreover the insulation and the surface smoothness of the substrateare improved.

An example of forming an insulation layer using a combination of apolymer film and an inorganic material film will be given below. Adimethylacetoamide solution of polyamide acid (solid content: 15% byweight) was applied on the cleaned surface of a stainless steel foilhaving a thickness of 0.1 mm. by means of a bar coater (clearance: 350μm.). After drying at 150° C. for 30 minutes, it was hardened at 300° C.for 30 minutes to form a polyimide film. The coated foil was placed in avacuum vapor deposition apparatus, and a SiO₂ layer was formed on thesurface of the polyimide layer by electron beam deposition (foiltemperature: about 300° C., vapor source: fused silica). The thicknessof the polyimide layer was about 20 μm. and the thickness of the SiO₂layer was about 3,000 angstroms. The thus obtained substrate had asufficient flexibility, and even if it was bent or deformed within thelimits of practical use, release and cracking of the insulation layerdid not occur. On the substrate, stainless steel was deposited in athickness of about 4,000 angstroms by sputtering to form a lowerelectrode. On the electrode, a photovoltaic device body having astructure of P-type a-Si/I-type a-Si/N-type a-SiC was formed by glowdischarge decomposition. An ITO transparent electrode having a thicknessof about 1,000 angstroms was then formed on the device body by electronbeam deposition. The thus fabricated flexible photovoltaic device hadthe following characteristics under a solar illumination power of 100mW/cm.² : short-circuit current density (Jsc)=14.5 mA/cm.², open circuitvoltage (Voc)=0.89 V, curve fill factor (FF)=60% and conversionefficiency (η)=7.74%.

Referring to FIG. 1, which shows an embodiment of a flexible thin filmsolar cell according to the present invention, the substrate isdesignated as 1. An electric insulating layer 1b such as a flexible,heat resistant polymer thin film or an inorganic material thin film isformed on a metal foil 1a. On the surface of the insulation layer 1b, alower electrode 2 which comes into ohmic contact with a P-type or N-typenon-single-crystal semiconductor is formed in a desired pattern in athickness of about 500 angstroms to about 1 μm. by deposition orsputtering. Examples of the material of the electrode 2 are metals suchas aluminum, molybdenum, stainless steel, nickel, platinum, gold andsilver and conductive metal oxides such as SnO₂, In₂ O₃ and ITO(indium-tin-oxide)(In₂ O₃ +SnO₂). On the surface of the electrode 2 is aphotovoltaic device body 3 composed of non-single-crystallinesemiconductor layers such as amorphous silicon semiconductor layers. Thephotovoltaic device body 3 is prepared in a thickness of 0.2 to 2 μm. bya plasma glow discharge method or a sputter deposition method. Atransparent electrode layer 4 which acts as a potential barrier to thephotovoltaic device body 3 and through which most of the light cantransmit, e.g. Pt, ITO, SnO₂, or a composite of ITO and SnO₂, is formedon the photovoltaic device body 3 by deposition in a thickness of 500 to3,000 angstroms. On the transparent electrode layer 4 is a comb-shapedelectrode grid 5 made of Al, Ag, Ti-Ag, Pd, or the like. Aanti-reflection layer 6, e.g. a zirconium oxide film, may be provided,as occasion demands. Also, if necessary, a passivation film is furtherprovided on the anti-reflection layer 6.

FIG. 2 is a schematic view of a vacuum apparatus for forming anamorphous silicon thin film and for fabricating the photovoltaic devicebody 3 including the amorphous silicon thin film layers by a plasma glowdischarge method or a sputtering deposition method. In FIG. 2, numerals8 and 9 indicate electrodes and numeral 10 represents a valve forintroducing a predetermined gas into a vacuum chamber 7. A substrate 1with a lower electrode 2 for the photovoltaic device is mounted on theelectrode 9 with the lower electrode 2 facing up. In the case of theplasma glow discharge method, after evacuating the chamber 7 to apressure of 1×10⁻⁶ to 10×10⁻⁶ Torr, a gas such as SiH₄ is introducedinto the chamber through the valve 10 at a pressure of 10⁻² to 5 Torrwith or without a dopant gas. A DC or RF voltage of 1 to several tens ofMHz is then applied between the electrodes 8 and 9, thereby causing aplasma to generate. This state is maintained for several tens ofminutes, during which the photovoltaic device body 3 including forexample an intrinsic amorphous silicon thin film layer having athickness of 0.1 to 1 μm., a carrier lifetime of not less than 10⁻⁷second, a density of localized states of not more than 10¹⁷ /cm.³ eV anda carrier mobility of not less than 10⁻³ cm.² /v.sec. is formed on thesurface of the electrode 2.

In the case of the sputtering deposition method, after evacuating thechamber 7 to a pressure of 1×10⁻⁶ to 10×10⁻⁶ Torr, a mixed gas ofhydrogen or the like and an inert gas such as argon is introduced intothe chamber through the valve 10 at a pressure of 5×10⁻³ to 10⁻¹ Torr. ADC or RF voltage of 1 to several tens of MHz is then applied between theelectrode 8 having a target thereon and the electrode 9 on which thesubstrate with the lower electrode 2 for the photovoltaic device ismounted, thereby causing a plasma to generate in the chamber. A voltageof about 1 to 3 kV, an electric current of 100 to 300 mA and a power of100 to 300 W are sufficient. If the above state is maintained for about60 minutes, the amorphous silicon photovoltaic device body 3 includingan intrinsic amorphous silicon thin film layer having a thickness of 0.1to 1 μm., a carrier lifetime of not less than 10⁻⁷ second, a density oflocalized states of not more than 10¹⁷ /cm.³ eV and a carrier mobilityof not less than 10⁻³ cm.² /V.sec is formed.

The term "non-single-crystal semiconductor" as used herein meansamorphous, semiamorphous and microcrystalline semiconductors. In thecase of employing such semiconductors in photovoltaic devices, a smallthickness is sufficient because they have a large optical absorptioncoefficient, and accordingly such semiconductors are suitable for use inflexible solar cells. The amorphous, semiamorphous or microcrystallinesemiconductor can be made of an element of Group IV of the PeriodicTable such as Si, Ge or Sn, or an amorphous, semiamorphous ormicrocrystalline multicomponent semiconductor made of at least twoelements selected from the group consisting of Si, Ge, Sn, N and C, andthe semiconductors in which the dangling bonds are terminated by anelement such as hydrogen or a halogen, are preferable. An amorphoussemiconductor and a microcrystalline semiconductor may be used incombination in forming a photovoltaic device body. In devicefabrication, structures such as Shottky barrier, PN, PIN, PINPINjunctions or combinations thereof can be used. Especially, devicesusing, an amorphous Si, SiN, SiGe, SiSn or SiC containing H or F as anI-layer are preferred. As occasion demands, semiconductors may bearranged in the order of large optical band gap in fabrication of PINjunction photovoltaic devices.

The lower electrode 2 for ohmic contact and the transparent electrodelayer 4 can be formed by a resistance heating method, electron beamdeposition method or sputtering deposition method.

Preferably, the photovoltaic device body 3 is of a three layerstructure. For instance, the first layer is a P-type amorphous siliconthin film having a boron concentration of not less than 10¹⁹ /cm.³, thesecond layer is an intrinsic amorphous silicon thin film, and the thirdlayer is a N-type amorphous silicon thin film having a phosphorusconcentration of not less than 10¹⁹ /cm.³ The first layer may be of aN-type conductivity and the third layer may be of a P-type conductivity.The short-circuit current and the open circuit voltage can be largelyimproved by employing at least as a window material a P-type or N-typeamorphous or microcrystalline silicon semiconductor having an opticalband gap of not less than about 1.85 eV, an electric conductivity of notless than about 10⁻⁸ (Ω·cm.)⁻¹ at 20° C. and a diffusion potential(exhibited in a PIN junction) of not less than about 1.1 volts. In thecase of using the above-mentioned photovoltaic device body 3 as a solarcell, the solar cell is constructed so that an open circuit voltage maybe obtained by radiating light from the side of the transparentelectrode 4 such as a Pt or ITO electrode, thereby causing electron-holepairs to be generated in the photovoltaic device body 3 by the light andcausing these electrons and holes to reach a depletion layer region.

In FIGS. 3A and 3B, representative examples of the basic construction ofthe photovoltaic device of the present invention are shown. Illustratedin FIG. 3A is a photovoltaic device of the type wherein light impingeson the P-layer side. For instance, the device of this type has theconstruction: stainless steel foil-insulatingfilm-electrode-N-I-P-transparent electrode. Illustrated in FIG. 3B is aphotovoltaic device of the type wherein light impinges on the N-layerside. For instance, the device of this type has the construction:stainless steel foil-insulating film-electrode-P-I-N-transparentelectrode. Optionally, other constructions may be formed by interposinga thin insulation layer or a thin metal layer between the P-layer orN-layer and the transparent electrode. Any construction suffices as longas a PIN junction is included as the basic component.

In the above PIN junction, there is employed, as I-layer, an intrinsicamorphous silicon (hereinafter referred to as "I-type a-Si") materialhaving a carrier lifetime of not less than about 10⁻⁷ second, a densityof localized states of not more than about 10¹⁷ cm.⁻³ eV⁻¹ and amobility of not less than 10⁻³ cm.² /V.sec., which is prepared by a glowdischarge decomposition of silane or its derivatives, fluorosilane orits derivatives, or a mixture thereof. Such an I-type a-Si layer isjoined with P-type and n-type doped amorphous or microcrystallinesilicon semiconductors to give a PIN junction structure. Preferably, atleast one of the P-layer and N-layer is a doped amorphous ormicrocrystalline semiconductor having an optical band gap of not lessthan about 1.85 eV, an electric conductivity of not less than about 10⁻⁸(Ω·cm.)⁻¹ at 20° C. and a diffusion potential (exhibited in the PINjunction) of not less than about 1.1 volts. Of course, such asemiconductor may be employed in both the P-layer and the N-layer. Thedoped layers are obtained by doping the above-mentioned I-type a-Si witheither an element of Group III of the Periodic Table to provide theP-type a-Si or an element of Group V of the Periodic Table to providethe N-type a-Si. If only flexibility is desired, a PIN homojunction maybe formed.

The P-type and N-type amorphous or microcrystalline semiconductors arenot particularly limited. Preferable amorphous or microcrystallinesemiconductors include amorphous or microcrystalline silicon carbide ofthe formula: a-Si.sub.(1-x) C_(x), amorphous or microcrystalline siliconnitride of the formula: a-Si.sub.(1-y) N_(y), amorphous ormicrocrystalline silicon carbon nitride of the formula: a-Si.sub.(1-x-y)C_(x) N_(y) and the hydrides and fluorides thereof. These amorphous ormicrocrystalline silicon compounds, hydrogenated or fluorinated, areprepared by glow discharge in a mixture of a hydrogen or fluorinecompound of silicon such as silane or fluorosilane, a hydrogen orfluorine compound of carbon or nitrogen such as a hydrocarbon orammonia, and a doping gas.

The above exemplified amorphous or microcrystalline semiconductors havea large optical band gap, and accordingly provide a very high opencircuit voltage (Voc), though it is naturally considered that theshort-circuit current density (Jsc) is increased when they are used as awindow material for a PIN junction photovoltaic device. In a PINjunction photovoltaic device using the above amorphous ormicrocrystalline semiconductors, there is a correlation between thediffusion potential (Vd) and the open circuit voltage of the device asdepicted by the band profile of FIG. 4. The trend of the relation isalmost the same without reference to the kind of the amorphous ormicrocrystalline semiconductor positioned on the light impinging side.The diffusion potential (Vd) is the difference obtained by subtractingthe sum of the activation energies of the P and N doped layers from theoptical band gap (Eg.opt) of the amorphous or microcrystallinesemiconductor positioned on the side exposed to the light. As shown inFIG. 4, let Ecn stand for the energy level of the conduction band on theN side and Evp for the energy level of the valence band on the P side,and the activation energy ΔEp and ΔEn can be determined based on thedependency of electric conductivity on temperature. Since ΔEp=Ef-Evpholds for the P-type and ΔEn=Ecn-Ef for the N-type, there ensueseVd=Eg.opt-(ΔEp+ΔEn). In the case of incidence of light on the N side,the diffusion potential is obtained in the same manner by subtractingthe Fermi levels (Ef) of the P and N layers from the optical band gap(Eg.opt) of the N-type semiconductor.

It is preferable to employ an amorphous or microcrystallinesemiconductor having an optical band gap of at least about 1.85 eV and adiffusion potential of at least about 1.1 volts. In the case offabricating a heterojunction photovoltaic device by employing such anamorphous or microcrystalline semiconductor, the short-circuit currentdensity (Jsc) and open circuit voltage (Voc) can be greatly improved.Also, an amorphous or microcrystalline semiconductor having an electricconductivity of at least 10⁻⁸ (Ωcm.)⁻¹ at room temperature is preferablyused, since when the electric conductivity is less than 10⁻⁸ (Ωcm.)⁻¹,the curve fill factor (FF) is small and no practical conversionefficiency is obtained.

The PIN heterojunction photovoltaic device provided according to thepresent invention will be described specifically below. In one typicalconstruction as shown in FIG. 3A, this device is of the type whereinlight impinges on the P-layer side, and is composed of metal foil 1a,electric insulating thin film layer 1b, electrode 2, N-type amorphous ormicrocrystalline silicon semiconductor 3a, I-type amorphous silicon 3b,P-type amorphous or microcrystalline semiconductor 3c and transparentelectrode 4. The transparent electrode 4 is desired to be formed of ITOand SnO₂, especially SnO₂, and it is directly formed on the P-typeamorphous semiconductor by vapor deposition. More preferably, thetransparent electrode is of two layer structure, and a SnO₂ layer havinga thickness of 30 to 100 angstroms is further provided in the interfacebetween the ITO layer and the P-type semiconductor layer. The P-typeamorphous or microcrystalline semiconductor layer 3c positioned on thelight impinging side is desired to have a thickness of from about 30 toabout 300 angstroms, especially from 50 to 200 angstroms. Although thethickness of the I-type a-Si layer is not specifically limited, it isgenerally selected from about 2,500 to about 10,000 angstroms. Also, thethickness of the N-type amorphous or microcrystalline siliconsemiconductor layer is not specifically limited, but is usually selectedfrom about 150 to about 600 angstroms. A heterojunction of amorphous ormicrocrystalline semiconductors may be used on the side opposite to thelight impinging side.

In another typical construction as shown in FIG. 3B, the device iscomposed of transparent electrode 4, N-type amorphous ormicrocrystalline semiconductor 3a, I-type amorphous silicon 3b, P-typeamorphous or microcrystalline semiconductor 3c, electrode 2, electricinsulating thin film 1b and metal foil 1a, and the transparent electrodeside is exposed to the light. For instance, a heterojunctionphotovoltaic device of this type has the construction: transparentelectrode--N-type a-Si.sub.(1-x) C_(x) :H--I-type a-Si:H--P-typea-Si:H-electrode--insulating layer--metal foil. The N-type amorphoussemiconductor on the light impinging side is desired to have a thicknessof from about 30 to about 300 angstroms, preferably from 50 to 200angstroms. Although the thickness of the I-type a-Si is not specificallylimited, it is generally selected from about 2,500 to about 10,000angstroms. The thickness of the P-type amorphous or microcrystallinesemiconductor layer, which is not particularly limited, is generallyselected from about 150 to about 600 angstroms. The material for thetransparent electrode and the method for the vacuum deposition thereofare the same as described above. Although a heterojunction of thesemiconductors is employed on the light impinging side in the abovedescription, a heterojunction of amorphous or microcrystallinesemiconductors may be employed on the opposite side to the lightimpinging side.

A plurality of generating zones connected electrically with each otherin series, each having the basic construction as shown in FIGS. 3A and3B, can be provided on a single substrate, namely an electricallyinsulated metal foil, for the purpose of providing a flexible,small-sized photovoltaic device capable of generating an increasedphotoelectromotive force.

Referring to FIGS. 5A, 5B and 5C, which show an embodiment of thephotovoltaic device of the invention having a plurality of generatingzones, numeral 11 is a metal foil, numeral 12 is an electric insulatingthin film layer, and numerals 13, 14 and 15 are first, second and thirdgenerating zones formed on the insulation layer 12 of the substrate.Each of the generating zones is composed of a homojunction orheterojunction semiconductor layer 16, a first electrode 17 and a secondelectrode 18, both electrodes being positioned on opposite sides of thelayer 16. The homojunction or heterojunction layer 16 has, for instance,the structure as shown in FIG. 3A and includes an N-type layer formed onthe substrate, a nondoped layer (i.e. I-type layer) and a P-type layer.The layer 16 may extend continuously throughout the first, second andthird generating zones, or may be discrete as shown in the figures.

The first electrode 17 is formed of a material which is capable of ohmiccontact with the semiconductor, e.g. a metal such as Ni, Cr, Pt or Moand a conductive metal oxide such as SnO₂, In₂ O₃, ITO (In₂ O₃ +xSnO₂,x≦0.1) or ITO/Ag. The first electrode 17 is preferably an ITO film onwhich a SnO₂ film having a thickness of 50 to 500 angstroms is formed.The second electrode 18 is a transparent electrode made of SnO, SnO₂,In₂ O₃ or a composite of SnO₂ and ITO formed on the SnO₂.

The first and second electrodes 17 and 18 of each of the first, secondand third generating zones 13, 14 and 15 have portions 19 and 20extending to the outside of the respective generating zones on thesubstrate. The extending portion 20 of the second electrode 18 of thefirst generating zone 13 overlaps the extending portion 19 of the firstelectrode 17 of the second generating zone 14 to electrically connectwith each other. Similarly, the extending portion 20 of the secondelectrode 18 of the second generating zone 14 and the extending portion19 of the first electrode 17 of the third generating zone are connectedto each other. A connecting portion 21 made of the same materials as thesecond electrode 18 may be attached to the extending portion 19 of thefirst electrode 17 of the first generating zone 13, as occasion demands.

The photovoltaic device as shown in FIGS. 5A to 5C is fabricated in thefollowing manner. On the substrate, namely the metal foil 11 insulatedwith the insulation layer 12, each first electrode 17 having anextending portion 19 is first formed by means of selective etching,selective sputtering or vapor deposition. In the second step, thehomojunction or heterojunction layer 16 is formed throughout the first,second and third generating zones. In that case, since the presence ofthe layer 16 on the extending portions 19 must be avoided, the layer 16is formed on only the desired portions by employing a mask covering theundesired portions, or after forming the layer 16 on the whole surfaceof the substrate provided with the electrode 17, the layer 16 on theundesired portions is removed by selective etching. In the next and laststep, the second electrodes 18 and the connecting portion 21 are formedby selective sputtering or vapor deposition technique.

If light enters into the homojunction or heterojunction layer 16 throughthe second electrode 18, a voltage is generated in each of the first,second and third generating zones. Since the generating zones 13, 14 and15 are connected to each other in series at the extending portions 19and 20 of the first and second electrodes 17 and 18, the voltagesgenerated at the respective zones are added, and the so added voltage isgenerated between the connecting portion 21 as negative electrode andthe extending portion 20 of the third generating zone as positiveelectrode.

If the distance between the adjacent generating zones is small, theremay be observed the phenomenon that an electric current flows directlybetween both first electrodes 17 of the adjacent zones, or both secondelectrodes 18 of the adjacent zones, namely, leakage current occurs.However, the occurrence of leakage current can be substantiallyeliminated by spacing the generating zones at a distance apart of atleast 1 μm., since the resistance of the homojunction or heterojuctionlayer 16 is from several to several tens of M at the time of lightimpingement. If necessary, the layer 16 may be separately formed inevery generating zone, and the first electrode may be connected inseries to the second electrode of the next adjacent generating zone.Also, it is advantageous for practical use to protect the photovoltaicdevice by covering the device from the second electrode side with anadherent transparent insulating polymer film or a transparent insulatingfilm such as SiO₂, amorphous silicon carbide, amorphous silicon nitrideor amorphous silicon carbon nitride film.

In the above-mentioned embodiment, a plurality of generating zones, eachincluding a homojunction or heterojunction photovoltaic device body, areconnected to each other in series, and accordingly a flexible,small-sized photovoltaic device capable of generating a desired voltagecan be provided. Also, since the insulating metal foil is employed as asubstrate, the photovoltaic device can be fabricated in the same manneras a conventional photovoltaic device using a glass substrate, moreoveron a mass production scale.

While particular embodiments have been described, it is to be understoodthat various changes and modifications may be made in the inventionwithout departing from the spirit and scope thereof.

What we claim is:
 1. A photovoltaic device comprising a substrateincluding a metal foil and an overlying electric insulating thin filmhaving an electric conductivity of not more than about 10⁻⁷ (Ω·cm.)⁻¹during light impingement and a photovoltaic device body ofnon-single-crystal semiconductors, the device body being formed oversaid electric insulating thin film, wherein said electric insulatingthin film includes a polymer layer contiguous to said metal foil and aninorganic material layer contiguous to an electrode for saidphotovoltaic device body.
 2. In an amorphous silicon solar battery ofthe type comprising a flexible and heat resistant substrate, at leasttwo lower electrodes formed on said substrate, an amorphous silicon filmformed on said lower electrodes, and at least two upper electrodes onsaid amorphous silicon film, the improvement which comprises aninorganic insulating film provided between said substrate and said lowerelectrodes and a heat resistant resin film provided between saidsubstrate and said inorganic insulating film.
 3. The amorphous siliconsolar battery according to claim 2 wherein said substrate is made ofmetal.